Trenchless engineering overview(1)

2012-09-19 14:02:04

DESCRIPTION
As the infrastructure in the China ages, increasing importance is being placed on rehabilitating the nation's wastewater treatment collection systems. Cracks, settling, tree root intrusion, and other disturbances that develop over time deteriorate pipe lines and other conveyance structures that comprise wastewater collection systems. These deteriorating conditions can increase the amount of inflow and infiltration (I/I) entering the system, especially during periods of wet weather. Increased I/I levels create an additional hydraulic load on the system and thereby decrease its overall capacity. In addition to I/I flow, storm water may enter the wastewater collection system through illegal connections such as down spouts and sump pumps. If the combination of wastewater, infiltration, and illegal storm water connections entering the wastewater treatment plant exceeds the capacity of the system at any point, untreated wastewater may be released into the receiving water. This bypass of untreated wastewater, known as a Sanitary Sewer Overflow (SSO), may adversely affect human health as well as impair the usage and degrade the water quality of the receiving water.
Under the traditional method of sewer relief, a replacement or additional parallel sewer line is constructed by digging along the entire length of the existing pipeline. While these traditional methods of sewer rehabilitation require unearthing and replacing the deficient pipe (the dig-and-replace method), trenchless methods of rehabilitation use the existing pipe as a host for a new pipe or liner. Trenchless sewer rehabilitation techniques offer a method of correcting pipe deficiencies that requires less restoration and causes less disturbance andenvironmental degradation than the traditional dig- and-replace method. Trenchless sewer rehabilitation methods include:
• Pipe Bursting, or In-Line Expansion;
• Sliplining;
• Cured-In-Place Pipe; and
• Modified Cross Section Liner.
These alternative techniques must be fully understood before they are applied. These four sewer rehabilitation methods are described further in the following sections.
Pipe Bursting or In-Line Expansion
Pipe bursting, or in-line expansion, is a method by which the existing pipe is forced outward and opened by a bursting tool. The Pipebursting™ method, patented by the British Gas Company in 1980, was successfully applied by the gas pipelines industry before its applicability was identified by other underground utility agencies. Over the last two decades, other methods of in-line expansion have been patented as well. During in-line expansion, the existing pipe is used as a guide for inserting the expansion head (part of the bursting tool). The expansion head, typically pulled by a cable rod and winch, increases the area available for the new pipe by pushing the existing pipe radially outward until it cracks. The bursting device pulls the new pipeline behind itself. The pipe bursting process is illustrated in Figure 1. Various types of expansion heads, categorized as static or dynamic, can be used on the bursting tool to expand the existing pipeline. Static heads, which have nomoving internal parts, expand the existing pipe only through the pulling action of the bursting tool. Unlike static heads, dynamic heads provide additional pneumatic or hydraulic forces at the point of impact. Pneumatic heads pulsate internal air pressure within the bursting tool, while hydraulic heads expand and collapse the head. While the dynamic head pulsates or expands and contracts, the bursting device is pulled through the existing pipeline and breaks up the existing pipe, replacing it with the new pipe directly behind it. Dynamic heads are often required to penetrate difficult pipe materials and soils. However, because dynamic heads can cause movement of the surrounding soils-resulting in additional pressure and ground settlement-static heads are preferred where pipe and soil conditions permit.
During the pipe bursting process, the rehabilitated pipe segment must be taken out of service by re¬routing flows around it. After the pipe bursting is completed, laterals are re-connected, typically with robotic cutting devices.